A cancer patient on long-term morphine develops tolerance that is more pronounced for analgesia than for constipation. Which cellular mechanism explains the differential tolerance?

• A Enteric neurons develop compensatory upregulation of mu-receptor expression over time
• B Morphine metabolite morphine-6-glucuronide selectively maintains GI effects
• C Neuronal mu-receptors in the brain and spinal cord undergo desensitization via GRK phosphorylation and beta-arrestin-mediated internalization; enteric neurons have lower GRK expression, maintaining constipation ✓
• D Serotonin receptor cross-desensitization occurs in CNS but not in enteric nervous system

Explanation

Opioid tolerance develops through receptor desensitization: GRK (G-protein receptor kinase) phosphorylation of the agonist-bound receptor followed by beta-arrestin binding, which uncouples the receptor from Gi/Go and promotes internalization. CNS neurons have robust GRK2/3 expression and undergo significant receptor desensitization and downregulation with repeated morphine exposure. In contrast, enteric neurons have lower GRK expression and different regulatory machinery, so mu-receptor coupling to inhibition of acetylcholine release in the myenteric plexus remains relatively intact. This explains why constipation persists while analgesic tolerance develops, justifying the use of peripheral mu-opioid antagonists (methylnaltrexone, naloxegol) for opioid-induced constipation without reversing central analgesia.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.